1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine for an automobile, an air-fuel ratio sensor is provided upstream of the three-way catalyst in the exhaust system for performing feedback control over the air-fuel ratio from the viewpoint of improving the cleaning efficiency of the three-way catalyst and improving the fuel efficiency. However, an air-fuel ratio sensor disposed upstream of the three-way catalyst is easily deteriorated due to the high temperatures there. Therefore, another air-fuel ratio sensor is provided downstream of the three-way catalyst for a so-called two sensor system, with the downstream sensor monitoring the control of the upstream sensor and correcting the same in accordance with the deterioration of the upstream sensor.
Even in a V type engine having two cylinder banks and two exhaust manifolds, use is made of two-sensor systems for the same purpose (see Japanese Utility Model Publication No. 63-79448, Japanese Unexamined Utility Model Publication No. 63-79449, and Japanese Unexamined Utility Model Publication No. 63-118354). In these systems, structurally, to eliminate one of the downstream air-fuel ratio sensors, the two exhaust passages are made to converge and a single auxiliary air-fuel ratio sensor is used for monitoring the feedback control of the air-fuel ratios of the cylinder banks performed by the two upstream main air-fuel ratio detecting sensors.
That is, the control system provided there, first, uses two main air-fuel ratio detecting sensors for performing feedback control over the air-fuel ratios of the cylinder banks independently so as to suitably adjust the exhaust and, second, uses a single auxiliary air-fuel ratio sensor for detecting the air-fuel ratio of the mixed exhaust of the two cylinder banks so as to correct the feedback control by the main air-fuel ratio detecting sensors from the detected value. For example, it may correct the air-fuel ratio by correcting the control values, i.e., the skip amount or the integration constant.
By this, even if the upstream main air-fuel ratio detecting sensors deteriorate and deviate in characteristics, this can be compensated for by the downstream auxiliary air-fuel ratio detecting sensor so as to ensure control to the target air-fuel ratio.
However, the downstream auxiliary air-fuel ratio detecting sensor is controlling the mixed exhaust to the target air-fuel ratio. Therefore, when the air-fuel ratio of the exhaust of one cylinder bank deviates in a reverse direction from the air-fuel ratio of the exhaust of the other cylinder bank, the deviations from the target air-fuel ratio of the air-fuel ratios of the two cylinder banks will cancel each other out and it will sometimes be judged that the air-fuel ratio of the mixed exhaust is the target air-fuel ratio.
When this situation arises, it becomes impossible for the air-fuel ratios of the cylinder banks to be restored to their normal states by the downstream auxiliary air-fuel ratio detecting sensor. Therefore, the air-fuel ratio continues deviated from the target air-fuel ratio and the cleaning performance of the catalyst quickly declines, resulting in the problems of a deteriorated fuel efficiency and emission.